Hi Edward & Günter,
I am more with Edwards point of view here—and I must have miss the message by
'Phil' you are referring to.
I do not see that you would get a thicker intermetalic layer in a solder joint
than in a large solder volume. On Cu we see Cu3Sn next to the Cu as a thin
layer and Cu6Sn5 as the thicker layer that grows with time and temperature.
NIST (formerly NBS) has done some work showing that on a free surface (cross-
sectioned solder joint) Cu3Sn actually grows faster than Cu6Sn5. Of course, in
a solder joint Cu3Sn does not have a free surface, the Cu is continuously
taken to make Cu6Sn5—on can view the Cu3Sn layer as a slowly moving diffusion
barrier for the Cu. Now the Cu6Sn5 has sort of a free surface to the molten
solder from which it graps Sn. This leads to a region outside the Cu6Sn5 that
is lead-rich. Thus, the growth of the Cu6Sn5 layer is slowed by the
'diffusion' rate  of the Cu through the Cu3Sn (staying constant in thickness)
and the Cu6Sn5 (growing in thickness) and the reduced availability of Sn. Of
course, once the Sn-rich phase is exhausted, growth stops altogether. This was
demonstrated by Magaret Nylen at the Swedish Metal Research Institute; she
used a Au-plated angeled gullwing lead and in the area with a thicker solder
gap all Au was dissolved and at the thinner gap not.
Agitation certainly will affect this situation and more Cu6Sn5 will be
produced, but whether that will lead to a thicker layer a more dispersal of
IMC throughout the solder volume I do not know.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]

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